1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device employing an active matrix drive system.
2. Description of the Related Art
Liquid crystal display devices are widely used as display device, not only in relatively small display capacity apparatuses such as small electronic desktop calculator and digital clock, but also in large display capacity apparatuses such as word processor or personal computer.
The display methods of liquid crystal display devices are classified by the shapes of the electrodes used in the display devices, into the segment type display suited to display of numerals, symbols and simple graphic patterns, and the matrix type display capable of displaying an arbitrary graphic pattern.
As the driving system of a liquid crystal display device having a segment type display, for example, a duty drive system or a static drive system may be employed. As a drive system of a liquid crystal display device having a matrix type display, for example, an active matrix drive system may be employed. In the active matrix drive system, switching elements and, if necessary, signal storing means are provided in pixels arranged in a matrix form, and the signals given to the wirings connected to pixels are supplied to each pixel and cut off individually by each pixel. As switching elements, for example, three-terminal elements such as a thin film transistor (TFT) and a field effect transistor (FET), and two-terminal elements such as a diode and a metal-insulator-metal (MIM) element are used. When the active matrix drive system is used in the twisted nematic (TN) type liquid crystal display device, a display screen of higher contrast is obtained, in comparison with the liquid crystal display device of different drive system from the active matrix driving system, for example, a liquid crystal display device of simple matrix driving system.
FIG. 12 is a sectional view showing a simplified constitution of a reflection type liquid crystal display device in the prior art. In this liquid crystal display device, a first polarizing plate 2 and a second polarizing plate 3 are provided on both sides of a liquid crystal panel 1. A reflector 4 is provided to be opposed to the liquid crystal panel 1 via the second polarizing plate 3. The transmission axes of the first and second polarizing plates 2, 3 are selected to be orthogonal or parallel to each other.
In the structure of the liquid crystal panel 1, between a pair of substrate members 5, 6, a TN type liquid crystal layer 7 in which direction of liquid crystal molecular axis is twisted by 90 degrees from one substrate member to the other substrate member is interposed. FIG. 13 is a partial plan view of one pixel of the liquid crystal panel 1. The region indicated by alternate long and two short dashes 10 is a region of one pixel.
The substrate member 5 comprises pixel electrodes 12 disposed in a matrix form on the surface of the substrate 11 facing the liquid crystal layer 7, signal wirings 13 disposed parallel to each other on every row or column of the pixel electrodes 12, and MIM elements 14 as switching elements for connecting the respective pixel electrodes 12 and signal wirings 13. Furthermore, an orientation film (not shown) is formed so as to cover the pixel electrodes 12, signal wirings 13, and MIM elements 14.
FIG. 14 is a partial sectional view of the substrate member 6. Referring to FIGS. 13 and 14, the substrate member 6 comprises counter electrodes 17 provided in a band form having a width confronting nearly an overall width of the pixel electrodes 12 in one row or one column in a direction orthogonal to the signal wiring 13, on the surface of the substrate 16 facing the liquid crystal layer 7, and scanning signal wirings 18 connected to the counter electrodes 17, which are arranged in parallel to the counter electrodes 17 at both ends of the respective counter electrodes 17 in a direction orthogonal to the longitudinal direction of the counter electrodes 17. Furthermore, an orientation film (not shown) is formed so as to cover the counter electrodes 17 and scanning wirings 18.
The transmission axes of the first and second polarizing plates 2, 3 are selected, for example, in the positive reflection type, so as to coincide with the orientation treating direction of the orientation films of the substrate members 6, 5 of the polarizing plates 2, 3 side. That is, the transmission axes of the first and second polarizing plates 2, 3 are orthogonal to each other. Therefore, in the state that voltage application is not carried out for the pixel electrode 12 and counter electrode 17 of one pixel on the liquid crystal panel 1, since the incident light from the first polarizing plate 2 side is propagated along the liquid crystal molecular axis in yhe liquid crystal layer 7 in the liquid crystal panel 1, the polarization direction of the light is changed by 90 degrees and the incident light passes through the second polarizing plate 3. The light is then reflected by the reflector 4, and gets into the liquid crystal panel 1 again. The polarization direction of the light is changed again by 90 degrees in the liquid crystal layer 7 in the liquid crystal panel 1, and the passes through the first polarizing plate 2 to exit.
When a predetermined voltage is applied to electrodes of the liquid crystal panel 1, electric fields are generated among the electrodes. At this time, the liquid crystal molecular array of the liquid crystal layer 7 existing in the electric field varies, and the liquid crystal molecular axis runs along the electric field direction, and hence optical rotary power for varying the direction of polarization of the light is lost. At this time, the direction of polarization of the light entered from the first polarizing plate 2 side is not changed, and hence the light cannot pass through the second polarizing plate 3. Accordingly, the light is not reflected by the reflector 4, and is not emitted from the first polarizing plate 2 side. Therefore, in the liquid crystal display device of the positive reflection type, displaying is achieved by the color of the display screen when voltage is not applied, that is, the white color when the light passes, and the color of the display screen when voltage is applied, namely, the black color when the light is blocked.
In the TN type liquid crystal display device, an art for preventing the contrast of the display image from varying depending on a viewing angle direction of the display screen is disclosed in Japanese Examined Patent Publication JP(B2) 4-14329 (1992). The viewing angle direction is the direction which a user observes the display screen, and it is expressed by the angle inclined from the vertical direction to the horizontal direction, relatively to the display surface of the device. In the prior art of this publication, the refractive index .DELTA.n of the liquid crystal material for forming the liquid crystal layer 7 and the thickness d of the liquid crystal layer 7 are selected so that their product .DELTA.n.multidot.d may be 210 nm to 600 nm.
The present applicant also proposed an art for improving the display contrast curve, in a multi-layer TN type liquid crystal display device using plural liquid crystal panels, in Japanese Unexamined Patent Publication JPA 58-130715 (1981). The liquid crystal display device of this prior art has plural TN type liquid crystal panels laminated and interposed between a pair of polarizing plates, in the light incidence direction. In this prior art, the transmission axes of the pair of polarizing plates or the directions orthogonal to the transmission axes are disposed by deviating the twist angle of the liquid crystal molecule in the decreasing direction by 3 degrees to 15 degrees, to the direction of the major axis of the liquid crystal molecule closest to the orientation film of the substrate member of the polarizing plate side in the liquid crystal panel closest to the substrates.
In the transmission type liquid crystal display device of the TN type, moreover, an art for heightening the contrast of the display screen and brightening the display screen is disclosed in Japanese Unexamined Patent Publication JPA 6-281927 (1994). In the liquid crystal display device of this prior art, a TN type liquid crystal panel of the same constitution as used in the reflection type liquid crystal display device in FIG. 12 is interposed between a pair of polarizing plates. In this device, light enters from one of the pair of polarizing plates, and the direction of polarization is changed in the liquid crystal panel, and display is made by the light passing through the other polarizing plate. In this prior art, the other polarizing plate at the light exit side of the pair of polarizing plates is a polarizing plate of high degree of polarization, and the one polarizing plate at the light incidence side is a polarizing plate of low degree of polarization.
In the liquid crystal panel 1 used in the prior arts, the counter electrode 17 contributing to actual display at the substrate member 6 side is realized by a transparent electrode such as ITO. As shown in FIG. 12, in the case where the reflector 4 is provided outside of the liquid crystal panel 1, the pixel electrodes 12 of the substrate member 5 side are also realized by transparent electrodes. The signal wirings 13 not directly contribute to actual display at the substrate member 5 side, but to which signals for display are supplied, scanning wirings 18 of substrate member 6 side, and HIM elements 14 of the substrate member 5 side are realized by a meal material of relatively small wring resistance so as not to cause delay of supplied signals, such as tantalum (Ta) and aluminum (Al).
However, these metal materials of small wiring resistance do not transmit light, and by disposing such metal materials on the substrates 11, 16, the TN type liquid crystal display device using this liquid crystal panel 1 is lowered in the aperture rate as compared with the TN type liquid crystal display device in which, for example, these wiring are also composed of transparent electrodes. The aperture rare refers to the ratio of the area of a light passing portion to the entire area of the display region of he device in the case of reflection type liquid crystal display device. The light passing portion includes, in the positive type, the confronting portion of the pixel electrode 12 and counter electrode 17 which is the portion contributing to the display in the display region, and the portion not confronting mutually to the display pixel electrode and counter electrode which are display electrodes, such as the gap between electrodes on the same substrate, that is, the always light passing portion regardless of application of voltage.
Namely, in the liquid crystal display device mentioned above, the incident light and the reflected light reflected by the reflector are blocked by the portion not transmitting light, and hence the transmittance and reflectance of the device are lowered. As a result, the brightness of the display screen drops, and the display quality of the liquid crystal display device deteriorates. In particular, the brightness of the background color is lowered.
As a method of raising the aperture rate of the liquid crystal display device, it may be considered to reduce the size of switching element or the like, or reduce the wire width of the metal wiring, but at the precision of the existing processing technique, when the size or wire width is further reduced, it is possible that breakage of metal wiring may be likely to occur. Further, when the size of the switching element is changed, the conditions about the design of the liquid crystal panel, such as the ratio of the element capacity and liquid crystal capacity may vary, and hence the design of the liquid crystal panel 1 must be largely revised.
In the reflection type liquid crystal display device, the light entered from outside passes through a pair of polarizing plates twice. The quantity of light passed through one polarizing plate is lowered to less than half as compared with the quantity of light before passing. Therefore, as compared with the transmission type liquid crystal display device designed to display by the light passed through a pair of polarizing plates only once, the light emitted from the reflection type liquid crystal display device is weak, and its display screen is dark. In the positive reflection type liquid crystal display device, moreover, the color of the background occupying the majority of the display area is white in the state of transmitting the light. As mentioned above, when displayed by the light lowered in the quantity of light, the background becomes dark. As a result, the entire display screen becomes dark, and hence the display quality further deteriorates. Therefore, as compared with the transmission type liquid crystal display device, it is demanded to increase the quantity of light emitted from the display screen and brighten the display screen. The prior art disclosed in the Japanese Unexamined Patent Publication JPA 6-281927 (1994) is an art to be applied to the transmission type liquid crystal display device, and when applied to the reflection type liquid crystal display device, the contrast and display screen are not improved to a practicable level.